1. Field of the Invention
The present invention relates to a cable for the transmission of electrical signals. Transmission performance is substantially improved over conventional cables for many applications, by the use of very pure silver as the conductive material, together with specific conductor cross-sectional shape and placement within the cable.
2. Description of the Related Art
A wide variety of cables are known and available, designed and made according to established, as well as theoretical principles of electrical signal transmission. The function of this type of cable in the most basic sense is simply to provide a conductive path for the signal to pass from one device to another. The cable will also, in most cases, establish contact between the grounded or zero voltage references of the devices being connected. This is often accomplished by the inclusion of a "shield", which is usually a tubular conductive material such as metallic foil, or a braid woven from strands of conductive material. The shield completely encloses the signal-carrying conductor or conductors and is electrically connected to ground potential at one or both ends of the cable. In this way, outside electromagnetic interference is substantially prevented from influencing the signal-carrying conductor, and the ground or reference connection between devices is established.
The most effective cable accomplishes the functions of signal transmission, ground connection, and shielding without adversely affecting the character of the signal. In other words, degradation of the signal due to the effects of resistance, capacitance, and inductance is minimized, although all electrical cables by their nature will exhibit all of these properties to some extent.
Upper frequency limit or effective bandwidth of a cable design is determined by the extent to which these properties can be eliminated.
One of the problems with conventional cables is that of the well known and documented "skin effect". This arises from the fact that the self inductance of a conductor is greatest at the center of the conductor, because the magnetic field set up by the changing current has the greatest rate of change at the center of the conductor. This means that higher frequency signals will encounter a lower impedance path toward the outside of the conductor and will concentrate there, reducing the effective cross-sectional area, and therefore increasing the impedance of the conductor for these frequencies. Thus, signals of higher frequency will encounter higher impedances than signals of lower frequency, and the relationships between amplitudes of different frequency signals being transmitted are distorted.
"Skin effect" has been documented at audio frequencies. Higher in the spectrum, at radio frequencies, virtually all of the current flows at the surface of the conductor, due to this phenomenon. Higher again at microwave frequencies, conductors are often hollow, because current flows only at the outermost surface. From the point of view of maximizing the linearity of signal transmission through a cable, any means of reducing this effect is central to the pursuit of increased bandwidth.
A similarly parasitic effect of bandwidth limitation is created by cable capacitance, since the proximity of the signal conductor and grounded conductors within the cable allows capacitive losses to occur, and again the problem increases with frequency.